The present invention relates to a heat shrink band steel sheet for tightening the panel of a color cathode-ray tube (CRT) used in televisions and the like and to a manufacturing method of it.
Since color CRTs are evacuated into a high vacuum of about 1xc3x9710xe2x88x927 Torr, the inevitable deformation of a panel surface by the atmospheric pressure has to be adjusted and the risk of the internal explosion of a tube must be avoided. For this purpose, so-called heat shrinking treatment is executed to applying tension for correcting the deformation of a panel surface by the following manner. That is, a heat shrink band composed of a steel sheet formed to a band shape is heated and expanded in the temperature range of about 400 to 600xc2x0 C. for several seconds to several tens of seconds; put over the panel of a color CRT; and then cooled and shrunk.
Further, since the heat shrink band has a function for shielding a geomagnetism similarly to the internal magnetic shield, it prevents the occurrence of landing error of electron beams on the surface of a fluorescent member, that is, the occurrence of color deviation which is caused by the geomagnetism.
Mild steel has been used as a material of heat shrink band. However, since the magnetic permeability of the mild steel at the level of the geomagnetism (about 0.3 Oe) is about 200 and the magnetic shielding characteristic of the mild steel is not sufficient, there is required troublesome processes such as the adjustment of the position of a fluorescent member, and the like to prevent the color deviation caused by the geomagnetism.
Proposed in Japanese Patent Laid-Open No. 10-208670 as a method of improving the magnetic permeability of a material for a heat shrink band at the level of the geomagnetism is to hot roll and/or cold roll steel, which comprises on the basis of percent in weight Cxe2x89xa60.005%, 2.0% xe2x89xa6Sixe2x89xa64.0%, 0.1%xe2x89xa6Mnxe2x89xa61.0%, Pxe2x89xa60.2%, Sxe2x89xa60.020%, sol Alxe2x89xa60.004% or 0.1% xe2x89xa6sol Alxe2x89xa61.0% and Nxe2x89xa60.005%; to anneal the thus rolled steel sheet at 700 to 900xc2x0 C.; and then to cold roll it at a reduction rate of 3 to 15%. It is shown that a heat shrink band having a magnetic permeability of at least 250 at 0.3 Oe and a sufficient magnetic shielding characteristic can be obtained by heating and cooling the steel sheet manufactured by the method.
However, when we actually applied the heat shrink band steel sheets made by the method disclosed in Japanese Patent Laid-Open No. 10-208670 to color CRTs, a sufficient magnetic shielding characteristic could not be always obtained.
An object of the present invention, which was made to solve these problems, is to provide a steel sheet for a heat shrink band having a sufficient magnetic shielding characteristic and capable of reliably realizing a color CRT with a less amount of color deviation and a manufacturing method of it.
The above object can be achieved by a steel sheet for a heat shrink band which comprises on the basis of percent in weight C: 0.1% or less, Si: 0.1% or less, Mn: 0.1 to 2%, P: 0.15% or less, S: 0.02% or less, sol Al: 0.08% or less, and N: 0.005% or less (hereinafter, the steel having the components is referred to as Steel 1), wherein the product of a magnetic permeability at the magnetic field of 0.3 Oe after heat shrinking treatment and a thickness (mm) is at least 350.
Further, a steel sheet for a heat shrink band which has a magnetic permeability, which is less deteriorated with aging can be obtained when the steel sheet for the heat shrink band comprises on the basis of percent in weight C: 0.005% or less, Si: 0.1% or less, Mn: 0.1 to 2%, P: 0.15% or less, S: 0.02% or less, sol Al: 0.08% or less, N: 0.005% or less, Ti: 0.02 to 0.06%, and B: 0.0003 to 0.005% (hereinafter, the steel having the components is referred to as Steel 2), wherein the product of a magnetic permeability at the magnetic field of 0.3 Oe after heat shrinking treatment and a thickness (mm) is at least 350.
A method of manufacturing the steel sheet having the components of Steel 1 comprises the steps of hot rolling and successively cold rolling the steel; annealing the cold rolled steel sheet in the temperature range of 650 to 900xc2x0 C.; and subjecting the annealed steel sheet to overaging treatment in the temperature range of 250 to 500xc2x0 C.
In contrast, in the case of the steel sheet having the components of Steel 2, it is preferable to anneal the cold rolled steel sheet in the temperature range of 800 to 900xc2x0 C. In this case, overaging treatment is not always necessary after the annealing. However, the deterioration with aging of the magnetic permeability can be considerably reduced after heat shrinking treatment by executing the overaging treatment in the temperature range of 250 to 500xc2x0 C.
The steel sheet can be skin-pass rolled after it is subjected to the overaging treatment or after it is annealed when it is not subjected to the overaging treatment likewise conventional steel sheets for the purpose of the improvement of the flatness of the steel sheet or the prevention of the occurrence of so-called stretcher-strain. In this case, a reduction rate must be set to 0.5% or less to prevent the deterioration of magnetic properties.